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This invention concerns a wiring method for producing a vertical integrated circuit structure. More specifically, the invention concerns a wiring method for vertical system integration.
A method for producing a vertical integrated circuit structure is known from German unexamined patent application DE 44 38 846 A1. In that method, first of all, the individual layers of components are processed on different substrates, independent of one another, and then joined together. First, via holes that go through all existing layers of the components are opened on the front of the top substrate. After that, the top substrate is thinned from the back as far as the via holes. Then, a finished bottom substrate is connected to the top substrate. After that, the via holes are extended as far as the metallization level on the bottom substrate (so-called interchip via holes), and, by filling the via hole with metallic material and structuring the metallic material on the surface of the chip stack so there is a connection between the via hole and the contact area on the uppermost metallization level, the contact between the top and bottom substrate is created (wiring).
The disadvantage of this method is, however, the fact that the integration density is not satisfactory.
The goal of this invention is therefore to provide a CMOS-compatible method for vertical system integration with freely selectable vertical contacts located between circuit structures of layers of components that have been joined together, which makes high integration density and low production costs possible. The goal of the invention is also to provide a vertical, integrated circuit structure with high integration density that can be produced simply.
This invention creates a wiring method for: producing a vertical integrated-circuit structure, with steps for providing a first substrate, which contains, in the first main area, one or more initial layers with circuit structures, and at least one uppermost metallization level with contact areas; opening via holes, in a first step, in the first main area of the first substrate; providing a second substrate, which contains, in the second main area, at least one layer with circuit structures and at least one metallization level; connecting the first substrate to the second substrate, so that the side of the first substrate opposite the first main area, and the side of the second main area of the second substrate are joined together in alignment; opening the existing via holes up to a predetermined metallization level of the second substrate; producing an electrically conductive connection between the first metallization level of the first substrate and the predetermined metallization level of the second substrate, so that via holes are opened in such a way that the via holes abut the contact areas of the uppermost metallization level of the first substrate.
This invention also creates a wiring method for producing a vertical integrated-circuit structure with steps for providing a first substrate that contains, in the first main area, one or more initial layers with circuit structures, and whose uppermost metallization level is not completed; for opening via holes in a first step in the first main area of the first substrate; for providing a second substrate that contains, in the second main area, at least one layer with circuit structures, and at least one metallizing layer; for connecting the first substrate to the second substrate, so that the side of the first substrate opposite that of the first main area and the side of the second main area of the second substrate are joined together in alignment; in a second step, opening the existing via holes as far as a predetermined metallization level of the second substrate; inserting a metallic material into the via holes; and selective metallization of the surface, as a result of which an uppermost metallization level of the first substrate is brought into contact with the predetermined metallization level of the second substrate by means of the metallic material in the via holes, in the course of which, the via holes are opened in such a way that the via holes abut the contact areas of the uppermost metallization level provided.
In the method in the invention, the individual layers of components are processed independent of each other on different substrates and then joined together. First of all, the substrate that has been finish-processed (the first substrate, hereinafter referred to as the top substrate), is equipped, at those points along the front with one or more layers of components and metallization levels, whose layers of structural elements, in the finished, vertically integrated microelectronic system, will be above the layers of the components of an additional substrate (second substrate, hereinafter referred to as the bottom substrate), with via holes at those points on the front at which vertical contact with the layers of components on the bottom substrate, which are under it, will be produced later.
The method differs from the state of the art known in unexamined patent application DE 44 38 846 A1, in that in the invention, the via holes penetrate structures of the uppermost metallization level. Preferably, they also penetrate all layers of components and metallization levels present in the top substrate. The via holes preferably end a few micrometers beneath the layers of the components on the top substrate, when an SOI substrate is used, preferably at the buried oxide layer. If the etching technique used here and later makes it necessary, the top substrate can be equipped with a so-called hard mask before the via holes are produced.
After the via holes are opened, the top substrate can be thinned from the back. Thinning can be done by wet chemical etching, and/or by mechanical, and/or by chemical-mechanical grinding, for example, so that, if necessary, the top substrate is mechanically stabilized with an auxiliary substrate applied to the front by means of an adhesive layer (handling substrate). In the process, the adhesive layer can have a passivating and/or planarizing function. However, thinning can also be done without using a handling substrate. For example, using current techniques, it is possible to thin the top substrate to a residual thickness of up to 50 xcexcm without using the handling substrate.
When using an SOI substrate, the buried layer of insulation can serve to good advantage as an etching stop during thinning. When using a conventional substrate (made of so-called bulk material), thinning can be done up to the via holes, so that afterward, they are open toward both sides of the top substrate.
Alternatively, it is naturally also possible to use a thin top substrate a priori, so there is no need for more thinning.
Then, another finish-processed substrate with one or more layers of components and metallization levels, the bottom substrate, is connected to the top substrate. For this, without imposing limitations in any general way, the front of the bottom substrate and/or the back of the top substrate can be equipped with a transparent adhesive layer. The adhesive layer can simultaneously perform a passivating and/or planarizing function. Then the top substrate and the bottom substrate are aligned to one another, and the back of the top substrate is connected to the front of the bottom substrate.
It is an advantage if alignment is done with split optics using alignment marks in the visible spectral range. Preferably, the alignment marks are made on the top substrate like the via holes by opening corresponding alignment structures from the front through all component layers of the top substrate. The alignment marks on the bottom substrate can be contained in the uppermost metallization level of the bottom substrate.
After the top and bottom substrate are joined together, the handling substrate that was used is removed, if necessary.
The existing stack of substrates can then be processed further as a standard substrate. The via holes already present are now extended from the front of the top substrate through the remaining layers (for example, the oxide layer of the SOI top substrate, the adhesive layer, the passivating layer of the bottom substrate) as far as the metallizing layer of a metallization level on the bottom substrate (by dry etching, for example), so that, if necessary, the hard mask of the top substrate serves as an etching mask. Finally, an electrical contact is made between the metal coating on the uppermost metallization level of the top substrate and the metallization level of the bottom substrate through these via holes.
For that purpose, this invention describes a new type of wiring method that leads to a marked increase in integration density, specifically for vertical system integration. Compared to the state of the art (DE 44 38 846 A1), in which wiring the stack of substrates to a metallization level of the bottom substrate by means of the via holes and wiring additional contact holes next to the stack to a metallization level of the top substrate must be done by means of additional printed circuits, the method in the invention makes the electrical connection by means of metal structures to be contacted that lie directly atop one another.
For this, the side walls of the via holes are preferably insulated initially by the deposition of SiO2, for example, and then the surface of the uppermost metallization level is exposed, thus creating contact surfaces for the metal structures on the uppermost metallization level which are penetrated by the via holes. This can be done, without imposing limits on the invention as a whole, by means of chemical etching (back etching), and/or by mechanical, and/or chemical-mechanical grinding (thus, without a lithographic step). Then, conductive material is deposited onto the surface and into the via holes of the substrate stack, and in one preferred embodiment, removed again from the surface of the top substrate by means of chemical etching and/or mechanical, and/or chemical-mechanical grinding, so that the via holes continue to be filled with conductive material (the so-called plug technique). Then, by means of a standard metallizing step, for example by depositing and structuring an aluminum alloy, subsequent passivation and opening of bond pads, the wiring of the substrate stack, and thus vertical system integration, is completed. In another embodiment, the contact to the uppermost metallization level of the top substrate is created after a corresponding lithography step by means of structured etching and a subsequent standard metallizing step.
A basic simplification of the wiring method for the vertical system integration described above can be achieved by connecting an incompletely metallized top substrate to the bottom substrate, provided that the component substrates to be integrated vertically are not finish-processed before stacking. In the process, production of the top substrate is stopped just before the processing of the uppermost metal level, and it is not concluded until after stacking the top and bottom substrate in one step, with wiring the filled via holes. Before connecting the top and bottom substrates, the via holes are opened at those points, in the case of the top substrate, at which the corresponding metal structures of the uppermost metallization level are later generated, so that the latter make direct contact with the via holes, which are filled with conductive material, when the stack of substrates is wired.